8. EMISSIONS

Emissions from animal waste
Emissions from synthetic fertilizer use
Emissions from enteric fermentation
Emissions from wetland rice cultivation

Emissions from animal waste

A number of estimates are available of the amount of nutrients in animal excreta. For example, the American Society of Agricultural Engineers (Midwest Plan Service, 1985) produced estimates of manure production for North American conditions, ECETOC (1994) produced estimates for European conditions and Bouwman et al. (1995) estimated worldwide excretion of nitrogen. The nitrogen excretion by animals is determined largely by the maintenance requirements of the animal, the amount of nitrogen in the milk production and the amount accumulating in meat. In general, extensive production systems are less efficient in N use than intensively producing systems. Therefore, nitrogen excretion does not increase proportionally with the production per head.

To estimate the current production of nitrogen by animals the excretion factors of Bouwman et al. (1995) are used (Table 17). The body weight and milk production largely determines the total feed intake (IPCC, 1995; Livestock Environment Study, in prep.). Estimates of the current body weights and feed intake averaged over the total population of an animal category are available for continents (IPCC, 1995). To approximate changes in body weights for non-dairy cattle, carcass weights were used as a correlate, in which the nitrogen excretion increases linearly towards a value between the current excretion in that region and the West European excretion. For dairy cattle the annual milk production per head is taken as a correlate for the nitrogen excretion, with a similar linear increase towards the European values. For the other animal categories constant N excretion rates were used, as these rates are not as variable for total regional populations as the N excretion of cattle (Bouwman et al., 1995).

TABLE 17
Nitrogen excretion, ammonia volatilization loss, feed intake and methane conversion factors for different animal categories

Category	Nitrogen excretion	NH3 loss	Feed intake	CH4 conversion
	(kg/head/yr)	(%)	MJ/day	(%)
Non-dairy cattle	40-45	20	60-130	6
Dairy cattle	60-100	30	100-250	6
Pigs	11	40	13-38	0.6-1.3
Sheep and goats	10	10	13-20	5
Poultry	0.5	40	-	-
Camels	55	20	100	7

Nitrogen excretion estimates based on Bouwman et al. (1995), NH₃ loss from RIVM-EDGAR (1995), feed intake and methane conversion from IPCC (1995).

The ranges of nitrogen excretion are given in Table 17. Although the body weight may not be proportional to carcass weight for non-dairy cattle, the relation used describes the increasing efficiency of nutrient use generally observed in developing livestock production systems (see, for example, ECETOC, 1994). To calculate the total nutrient excretion (N + P₂O₅ + K₂O), factors specific for animal categories from Midwest Plan Service (1985) were used. According to the medium scenario for the period 1990-2010 the populations of the different animal categories in the developing countries will continue the fast increase observed in the past decades (Table 18). The total excretion calculated on the basis of the above assumptions varies between regions depending on the population and productivity scenarios of the different animal categories (Figure 7). In most regions, however, the animal excretion is dominated by cattle. The total nutrient excretion in all developing countries showed a fast increase of 1.75% per year in the period 1960-1990; the increase in the medium scenario will continue at a fast rate of 1.5% per year in the period 1990-2010, and somewhat slower in 2010-2025 (1.2% per year).

After 2025 the excretion will stabilize at a level of 250 million ton (Figure 7h). Comparison of the different scenarios shows that the low scenario results in higher, and the high scenario in lower waste production per unit product than the medium scenario (Figure 8). This results largely from the assumptions on the efficiency of nutrient use in the different scenarios. Emissions of NH₃, N₂O and CH₄ from animal waste have been estimated.

Ammonia (NH₃)

The volatilization of NH₃ from animal wastes depends on whether the waste is dropped in a stable or in the field. The type of stable is important for the NH₃ losses, as well as the management of the waste. Field emissions are determined by many factors, of which wind speed, temperature and rainfall are the most important. In their global inventory Bouwman et al. (in prep.) used NH₃ volatilization rates of 15-20% for stable and storage conditions, 25% for NH₃ loss for manure application as fertilizer, and 10-15% for grazing conditions. The resulting estimates for the NH₃ losses from animal waste vary from 40% for poultry to 20-30% for cattle (Table 17).

TABLE 18
Populations of different animal categories for three different scenarios. M, H, L = medium, high and low scenario. Populations in million heads

Category		'61/63	'69/71	'79/81	1989BY	'89/91	2010	2025	2050	2075	2100
Cattle	M	689	796	909	1005	1020	1299	1431	1428	1283	1235
	H						1072	1143	1073	942	909
	L						1487	1697	1816	1720	1704
Dairy cattle	M	86	100	131	158	160	197	252	306	279	239
	H						158	172	171	150	144
	L						224	310	427	450	452
Pigs	M	171	291	435	486	500	857	984	902	846	891
	H						807	865	788	703	712
	L						929	1142	1209	1323	1460
Sheep and goats	M	760	842	982	1129	1136	1478	2192	3286	4084	4875
	H						1352	1568	1680	1596	1529
	L						2001	2947	4446	5549	6646
Poultry	M	1851	2470	3757	6469	6770	10629	13968	16893	19834	20509
	H						9261	12039	13983	15033	15038
	L						12296	17357	22769	26696	27936

FIGURE 7
Estimated current and future animal nutrient excretion for the medium scenario from all animal categories. Excretion in million ton N + P₂O₅ + K₂O.

a. East Asia

b. China and C. P. Asian countries

c. South Asia

d. Near East in Asia

e. North Africa

f. Sub-Saharan Africa

g. Latin America

h. All developing incl. China

TABLE 19
Ammonia emission (a) and nitrous oxide emission (b) from animal excreta for developing countries for 1990. Emissions in million metric tons N per year

Region	1960	1970	1980	1990	2010	2025	2050
a. NH3-N
East Asia	0.3	0.4	0.4	0.6	0.9	1.0	1.2
China and C.P. Asian countries	1.2	1.9	2.5	3.4	5.7	7.4	7.4
South Asia	2.8	3.0	3.5	3.9	5.3	6.1	6.9
Near East in Asia	0,5	0.5	0.6	0.6	0.8	1.1	1.4
North Africa	0.1	0.2	0.2	0.3	0.4	0.4	0.5
Sub-Saharan Africa	1.2	1.5	1.8	2.1	2.7	3.6	4.7
Latin America	2.1	2.5	3.3	3.7	4.4	4.4	4.2
Developing	8	10	12	15	20	24	26
Developed				8
World				23
b. N2O
East Asia	0.0	0.0	0.0	0.0	0.0	0.0	0.1
China and C.P. Asian countries	0.1	0.1	0.1	0.1	0.2	0.3	0.3
South Asia	0.1	0.1	0.2	0.2	0.2	0.3	0.3
Near East in Asia	0.0	0.0	0.0	0.0	0.0	0.1	0.1
North Africa	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Sub-Saharan Africa	0.1	0.1	0.1	0.1	0.1	0.2	0.2
Latin America	0.1	0.1	0.1	0.2	0.2	0.2	0.2
Developing	0.4	0.5	0.6	0.6	0.9	1.0	1.2
Developed				0.4
World				1.0

Emission calculations based on RIVM-EDGAR (1995).

FIGURE 8 - Estimated current and future animal nutrient excretion from all animal categories and synthetic fertilizer use in all developing countries including China. Excretion and fertilizer in N + P₂O₅ + K₂O.

Because in most regions the excretion is dominated by cattle, the total regional NH₃ loss can be expected to be close to that of cattle. The current emission of NH₃ associated with the ~ 100 million ton N in animal waste is about 23 million ton NH₃-N per year (Table 19). For developing countries including China, the NH₃ emissions amount to about 16 million ton NH₃-N per year from the 70 million ton of nitrogen in excreta. The percentage of NH₃ loss from animal excreta in developed countries is very close to that in developed countries (Bouwman et al., in prep.). This indicates that the productivity level and the animal waste management have minor influence on the NH₃ losses for large regions of the world.

Therefore, in the emission scenario the NH₃ loss rates are assumed to be constant in time, and the nitrogen excretion varies according to the above assumptions (Table 17). It must be noted that considerable reductions in NH₃ losses can be achieved for animal excreta applied as fertilizer to arable land¹. In the medium scenario the NH₃ emission from animal waste in the developing countries will increase considerably from current 15 million tons NH₃-N per year to 20 million tons in 2010, and 24 million tons in 2025. After 2025 the emissions increase at a much slower rate to 26 million tons in 2050. (Table 19).

¹ A higher N recovery can be achieved from reduction of NH₃ volatilization by incorporating the manure. At the same time the animal manure substitutes part of the synthetic fertilizer.

Nitrous oxide (N₂O)

Very few measurement data are available from literature for nitrous oxide (N₂O) emissions from animal waste. Therefore, the emission rate of 1% proposed by Bouwman et al. (1995) is used for both grazing and confined animals². The annual emission of N₂O from animal excreta also showed a rapid increase from 0.4 million ton N₂O-N in 1960 to 0.6 million ton in 1990, and further to 1.2 million ton in 2050 (Table 19). In comparison, the annual N₂O emissions estimated for 1990 for the developed countries were 0.4 million tons (Table 19).

² This simple method does not account for differences in animal waste management systems. The animal waste management is an important factor regulating N₂O emission rates. For example, low emission rates of <0.01% of the N from anaerobic digesters can be expected, and high emissions of up to 15% N from deep litter stables (IPCC/OECD preliminary report of the IPCC/OECD working group on N₂O emissions from agriculture [in prep]. This group is preparing more default estimates of N₂O from grazing animals and animal waste management systems, including animal manure applied as fertilizer).

Methane (CH₄)

The emission of CH₄ from animal waste depends on the waste management system. Highest CH₄ emissions occur where animal waste is stored in lagoons, or where the waste is stored in liquid form or as a slurry. These systems are not widespread in the developing countries, except in parts of Asia. In the developing countries most of the non-dairy cattle are grazing animals. In dairy production both grazing and daily spreading of manure are widespread. Much of the pig manure in Asia is stored in liquid form.

Of the global emission of 10-15 million ton CH₄ per year, the developing countries may contribute some 30%. The emission rates can be assumed constant in the future scenarios, and then the emission from developing countries would increase from the current ~ 4 to perhaps ~7 million ton CH₄ per year in 2050. However, the amount of waste produced in developing countries will increase in all scenarios, as discussed above. This may create waste disposal problems, whereby other systems with high emission rates (such as lagoon or liquid storage systems) may increasingly be adopted. With the increase of animal waste production resulting from the scenarios (Figures 8 and 9), this may lead to a significant increase in the emission from developing countries. Assuming that the emission rates for the year 2050 are equal to those used for current European emissions by IPCC (1995), the emission in 2050 from developing countries would perhaps be 50 million tons CH₄ per year, which is close to the estimated 80 million ton CH₄ from enteric fermentation in 2050 (Figure 10).

FIGURE 9
Comparison of fertilizer use and animal nutrient excretion for the medium scenario. Nutrients in million ton N + P₂O₅ + K₂O.

a. East Asia

b. China and C. P. Asian countries

c. South Asia

d. Near East in Asia

e. North Africa

f. Sub-Saharan Africa

g. Latin America

h. All developing incl. China

Emissions from synthetic fertilizer use

The scenarios of synthetic fertilizer use presented in Chapter 6 can be used to produce estimates of emission scenarios of nitrous oxide. The fraction N in of the total NPK use as presented in Appendix 18 does not show much variation over the period 1960-1990. Therefore, the percentage N in the NPK fertilizer used in the fertilizer scenario is the average 1960-1990 level. On the basis of the NPK scenario and the assumed constant N fraction, the N fertilizer use calculated for the developing countries will increase from the current 41 million ton N per year to 77 million ton in 2010, 105 million ton in 2025 and close to 140 million ton in 2050 for the medium fertilizer scenario variant.

The estimated global emission of NH₃ from current synthetic N fertilizer use is about 9 million ton N per year. Most of this occurs in the developing countries (7.5 million ton N), and about half in China. The average N losses as ammonia from synthetic fertilizer use of 18% in developing countries is much higher than that in developed countries (5%). The loss rates are related to the type of fertilizer and to the climatic conditions. In addition, the NH₃ emission may be higher in wetland rice cultivation than in dryland fields. In developing countries about 50% of the nitrogen fertilizer used is in the form of urea (IFA, 1994). Asman and Bouwman (1995) indicated that NH₃ losses from urea may be 25% in tropical regions and 15% in temperate climates. In China 40-50% of the nitrogen fertilizer used is in the form of ammonium bicarbonate (Asman and Bouwman, 1995), which is a highly volatile compound³. The NH₃ loss from ammonium bicarbonate may be 30% in the tropics and 20% in temperate zones. Contrary, the NH₃ loss from injected anhydrous ammonia, which is widely used in the USA, is only 4% (Asman and Bouwman, 1995).

³ Urea is less volatile than ammonium bicarbonate. In the soil urea is converted to ammonium bicarbonate by the enzyme urease, which takes about 2-3 days.

It is obvious that future NH₃ emissions depend very much on the type of fertilizer. The mix of N fertilizers may change in the future, so that the average NH₃ loss rate may also change. Here, it is assumed that all developing countries achieve a reduction of NH₃ losses to a rate of 5%, equal to the current loss rate for developing countries. The N fertilizer thus saved would be 5 million ton N per year, which is 13% of the current fertilizer N use in developing countries and 7% of current global N fertilizer use⁴. If the loss rate of 5% is applied to the future scenarios of fertilizer use, the NH₃ loss from fertilizers will still be lower in 2025 than the current emission (Table 20).

⁴ The saving in fertilizer represents about US$ 2 billion at current prices, using the October 1995 price of urea. The saving of 5 million ton N is the equivalent of the nitrogen content of 250 million ton rice, about equal to the total Chinese rice consumption in 1990.

Nitrous oxide emissions amount to 1.25 ± 1 % of the nitrogen applied. This estimate is the average for all fertilizer types, as proposed by Bouwman (1995) and adopted by IPCC (1995). The emission rates vary from one fertilizer type to another (Bouwman, 1995). For example, the N₂O loss from injected anhydrous ammonia is generally much higher than losses evolving from nitrate-based fertilizers. However, the data set available from measurements published in the literature is too limited to produce reliable estimates of emission rates by fertilizer type (Bouwman, 1995). According to the medium fertilizer scenario this loss of 1.25% results in an increase from the current 0.5 million ton N₂O-N to 1.0 million ton N in 2010, and further to 1.7 million ton N in 2050 for all developing countries including China (Table 21).

FIGURE 10
Estimated current and future methane emission from all animal categories for the medium scenario. Methane emission in million ton CH₄ per year.

a. East Asia

b. China and C. P. Asian countries

c. South Asia

d. Near East in Asia

e. North Africa

f. Sub-Saharan Africa

g. Latin America

h. All developing incl. China

TABLE 20
Synthetic N fertilizer use in 1990 and NH₃ emission from fertilizers

Region	N fertilizer use in 1990 (Mton N)	NH3 loss in 1990 (%)	NH3 loss in 1990 (Mton N)	NH3 loss of 5% (Mton N)
				1990	2010	2025
East Asia	3.4	19	0.6	0.2	0.3	0.4
CP Asia	20.3	18	3.7	1.0	1.6	2.0
South Asia	9.8	21	2.1	0.5	1.1	1.6
Near East Asia	2.4	13	0.3	0.1	0.3	0.4
North Africa	1.1	12	0.1	0.1	0.1	0.1
Sub-Saharan Africa	0.7	10	0.1	0.0	0.1	0.2
Latin America	3.8	15	0.6	0.2	0.4	0.5
Developing	41	18	7.5	2.1	3.9	5.3
Developed	34	5	1.7	1.7
World	76	12	9.2	3.8

TABLE 21
Emission of N₂O caused by synthetic fertilizer application. Emission in million ton N₂O-N per year

Region	1960	1970	1980	1990	2010	2025	2050
East Asia	0.0	0.0	0.0	0.0	0.1	0.1	0.1
China and C.P. Asian countries	0.0	0.0	0.2	0.3	0.4	0.5	0.6
South Asia	0.0	0.0	0.1	0.1	0.3	0.4	0.5
Near East in Asia	0.0	0.0	0.0	0.0	0.1	0.1	0.1
North Africa	0.0	0.0	0.0	0.0	0.0	0.0	0.1
Sub-Saharan Africa	0.0	0.0	0.0	0.0	0.0	0.1	0.1
Latin America	0.0	0.0	0.0	0.0	0.1	0.1	0.2
Developing	0.0	0.1	0.3	0.5	1.0	1.3	1.7
Developed	0.1	0.3	0.4	0.4
World	0.2	0.4	0.8	1.0

Emissions from enteric fermentation

The current CH₄ emission associated with enteric fermentation by cattle, pigs, sheep and goats, and camels, is calculated with the method described in IPCC (1995). Future emission rates are estimated on the basis of similar assumptions to those used to estimate nitrogen excretion. The animal body weight is assumed to determine the future energy intake, and carcass weight is used as a correlate for the body weight for all animals, except dairy cattle, where milk production is assumed to determine the energy intake. The CH₄ emission is calculated from the energy intake as follows: emission (kg/head/yr) = GEI x Y / EC, where GEI is the gross energy intake (MJ/head/day) (Table 17), Y the methane conversion rate in decimal form (Table 17), and EC the conversion factor to compute methane emission (EC = 55.65 MJ/kg of methane). The range of gross energy intake estimated for the different animal categories (Table 17) shows much more variability than the nitrogen excretion ranges. This reflects the observed large differences in the CH₄ emission per animal between intensive and extensive production systems (IPCC, 1995).

FIGURE 11 - Estimated current and future methane emission from all animal categories for three scenarios in all developing countries, including China

The CH₄ emissions by animal category for the different regions (Figure 10) show that cattle is by far the major source of CH₄ from enteric fermentation. Latin America and South Asia, with their large cattle populations, are important contributors to the total emission from all developing countries. The current contribution to the global emission from all developing countries is about 50%. The emission of CH₄ from enteric fermentation from developing countries increases in the medium scenario by about a factor of 2. Comparison of the different scenarios shows that the low scenario results in higher, and the high scenario in lower total emission than the medium scenario (Figure 11). This result is very similar to the scenarios of nutrients in animal waste, and is caused by the higher feed use efficiency in more intensive production systems.

Emissions from wetland rice cultivation

In the most recent IPCC assessment (Prather et al., 1995) rice cultivation is considered to be a minor global source of CH₄, contributing less than 10% and probably only about 5% to the global emission of about 500 million ton CH₄ per year. More than 90% of the global rice production occurs in developing countries. No attempt has been made to estimate future emissions. However, the general tendency in the medium scenario is a nearly constant harvested area of rice. If the CH₄ emission per hectare of rice does not change, the source strength will, therefore, not change in the medium scenario (Figure 12). The global rice area will, however, increase in the low scenario. If the CH₄ emission is related to the biomass production, an important increase of the global emission can be expected because the global rice production will double between 1990 and 2050.

FIGURE 12
Results of the scenario for rice cultivation

a) total, rainfed and irrigated harvested areas (in million ha) for medium scenario;

b) index of production and area for medium scenario, 1990 = 100.